In the field of nuclear analysis such as a positron lifetime spectrometer or a positron angle-momentum association analyzer, the field of nuclear detection such as double-coincident high-energy-particle discrimination and the field of medical imaging such as positron emission tomography (abbreviated as PET), a detector has two operating principles. That is, one operating principle is to convert, by a scintillator, a high-energy photon into a visible photon or an ultraviolet photon having low energy, and then convert, by a photoelectric device, the visible photon or the ultraviolet photon into an electrical signal; and the other operating principle is to directly convert a high-energy photon into an electrical signal by a semiconductor material such as Cadmium Zinc Telluride (abbreviated as CZT). The detector outputs the electrical signal in the two operating principles described above.
In a PET system, system performance is improved and an application scope is extended in a case of good time resolution. First, in a case that the time resolution is good enough (for example, less than 800 picoseconds), a location where positron annihilation occurs is deduced based on a time difference of the arrival of two electrical pulses, a value of the location meets the Gaussian distribution, and the full width half maximum of the distribution is less than 12 cm (corresponding to 800 picoseconds). Information on the location has a significant effect on improving a signal-to-noise ratio of an image. Secondly, the good time resolution can facilitate rejecting scattering better, and improving system noise equivalent counting. Thirdly, since the time difference has a positioning ability for a coincident event along a direction of a line of response (abbreviated as LOR), a completeness requirement for a projection data may be reduced by reestablishing a PET image having time information, and thus an image can be reestablished with incomplete data. Again, attenuation data and emission data can be acquired simultaneously in the PET system having the time information, to shorten a scanning time period, and reduce complexity of a hardware system. Also, multiple mice can be imaged respectively at the same time in the system, and aliasing is prevented.
In order to improve the time resolution of the system, there are three normal methods, that is, a method a, a method b and a method c. The method a is to select a crystal having fast attenuation. The method b is to select a photon multiplier tube having small transit time spread and high quantum efficiency. The method c is to optimize a time label method. The method a and the method b are given, the method c is a concerned issue in the art.
A leading edge discrimination (abbreviated as LED) is used as a simplest time label method for acquiring time of arrival of a pulse in a PET data acquiring system. A reference voltage is set, and time when a voltage amplitude of a pulse exceeds the reference voltage represents time of arrival of a signal event. The method is widely used in a case that a rising edge of a processing pulse is steep and a change in the amplitude is small since the method is easy to be implemented and time jitter caused by noise is small. The method has disadvantages that time walk occurs since the method is vulnerable to the amplitude of the pulse and the rise and fall of a slope of the rising edge, thereby reducing accuracy of the time label.
In order to eliminate the time walk due to the amplitude of the pulse, a constant fraction discrimination (abbreviated as CFD below) is set forth, in which, a scintillation pulse includes two signals. One signal is attenuated and reversed at an attenuation terminal of the CFD, and the other signal is delayed for a constant time period at a delay terminal of the CFD. The delayed signal and the attenuated and reversed signal are added to generate a bipolar signal, and a zero-crossing point of the bipolar signal is detected by a zero-crossing discrimination in the CFD. A time instant of the zero-crossing point is time of arrival of a time label event of the CFD. The delay time period and an attenuating proportion in the CFD are preferred, a timing error caused by the amplitude of the pulse and rising time fluctuation is eliminated by the CFD, and therefore good time performance can be obtained for the PET data acquiring system.
Whether the LED method or the CFD method is developed based on an analog circuit in a conventional time acquiring system. Performance parameters of the analog circuit drift with a change in time, a temperature and an operating environment, and it is difficult to maintain the analog circuit in a state of high performance in an actual system. Specifically, it is a huge challenge to correct the performance parameters for a system such as the PET having thousands of detection channels.
With the rapid development of digital technology the digital leading edge discrimination (abbreviated as DLED below) and the digital constant fraction discrimination (abbreviated as DCFD below) have gradually become an important time label method. The two digital time label methods can be flexibly implemented in a digital device such as a field programmable logic array (abbreviated as FPGA below), a digital signal processor (abbreviated as DSP below). However, their performances are limited by a sampling ratio of an analog-to-digital convertor (abbreviated as ADC below) to a great extent, since an existing PET detector is inclined to select a scintillation crystal having a small attenuation time constant and an photon multiplier tube (abbreviated as PML below) having a fast rising time period to acquire good time performance and counting ratio performance. Taking a mainstream scintillation detector such as LSO/PMT as an example, a rising time period of a scintillation pulse signal outputted from the scintillation detector is normally in a range from 1 ns to 20 ns, and duration of the pulse is 200 ns. In order to achieve time performance the same as or similar to that of the CFD method in a case that time of arrival of the pulse is acquired by the DCFD method and no filtering processing is performed on the scintillation pulse, a sampling ratio of the ADC used in the DCFD method is at least up to 1 Giga samples per second (abbreviated as GSPS below). However, it is no doubt that the high sampling ratio of the ADC brings up troubles of high cost, ultra-high data throughout and ultra-high data processing for the PET. Similarly, a digital pulse time extraction method based on the ADC sampling, such as the mean PMT pulse model (abbreviated as MPPM below), the maximum rise interpolation (abbreviated as MRI) and the initial rise interpolation (abbreviated as IRI below) may also get into a conflict between a high sampling ratio requirement and a high time resolution performance.
Therefore, with regard to the technical problems described above, it is necessary to provide a new time label combination method and system for data volume which can be acquired, to overcome the disadvantages described above.